Power devices are widely used in various fields and in addition to being used as inverters for controlling motors, are used as power supplies for display panels such as large-capacity plasma display panels (PDPs) and liquid crystal display panels, and as inverters for home appliances such as air-conditioners and lighting fixtures. The driving and control of such power devices has been carried out by an electronic circuit formed by combining electronic parts such as semiconductor devices like photocouplers and transformers. However, recent advancements in large-scale integrated circuit (LSI) technologies have enabled practical use of high voltage ICs of up to 1200V, facilitating high voltage ICs that integrate power semiconductor devices as high side gate drivers and low side gate drivers as well as those that integrate a control circuit and a power semiconductor device on the same semiconductor substrate, thereby contributing to greater efficiency, and a reduction in the number of parts and in mounting area.
FIG. 13 is a circuit diagram of a high voltage IC having an ordinary level shift circuit. In FIG. 13, diodes 41 and 42 are added to the circuit shown in FIG. 8 of Patent Document 1. In FIG. 13, reference numerals 17 and 18 denote IGBTs (output power devices) connected in series between a positive electrode side terminal Vdc of a main DC power supply of a high voltage of 400V, for example, and a common electric potential point COM (the ground in FIG. 13) as a negative electrode side of the power supply to form one phase of, for example, an electric power inverting bridge circuit of a PWM inverter. An OUT terminal is a connection point of the emitter of the IGBT 17 in the upper arm of the bridge circuit and the corrector of the IGBT 18 in the lower arm of the bridge circuit. The OUT terminal is also an AC output terminal for AC electric power generated by the alternate turning-on and -off of the IGBTs 17 and 18.
The sign E2 denotes an auxiliary power supply (also referred to as a driver power supply) whose positive electrode is connected to a positive electrode line Vcc2 and negative electrode is connected to a common electric potential point COM and that supplies a low voltage of, for example, 15V. Reference numeral 20 denotes a driver that carries out on and off driving of the IGBT 18 in the lower arm and is operated by the auxiliary DC power supply E2.
Other components in the circuit form a level shift circuit for driving the IGBT 17 in the upper arm of the bridge circuit. Reference numerals 1 and 2 denote a high voltage MOSFET. The high voltage MOSFET 1 conducts with the input of an on-signal 25. The on-signal 25 is a pulse signal generated by a control circuit 61 (low electric potential side, low blocking voltage circuit) to which a current is supplied by a low voltage power supply whose reference electric potential is that on the negative electrode side of the main DC power supply (the COM electric potential). Conduction by the high voltage MOSFET 1 causes a voltage drop at the connection point of the high voltage MOSFET 1 and a load resistor 3. With the voltage drop taken as a signal, the IGBT 17 is turned-on. Similarly the high voltage MOSFET 2 conducts with the input of an off-signal 26 that is a pulse signal generated by the control circuit 61. Conduction by the high voltage MOSFET 2 causes a voltage drop at the connection point of the high voltage MOSFET 2 and a load resistor 4. With the voltage drop taken as a signal, the IGBT 17 is turned-off.
Here, in general, the high voltage MOSFETs 1 and 2 are equivalent and the load resistor 3 and 4 are also equivalent. Moreover, voltage-regulator diodes 5 and 6 connected in parallel to the load resistors 3 and 4 restrict excessive voltage drops across the load resistors 3 and 4 play roles in respectively protecting components such as NOT circuits 8 and 9, explained hereinafter. Among the level shift circuits, the two high voltage MOSFETs 1 and 2 are circuit sections to which signals are input having the electric potential at the static common electric potential point COM as a reference.
The circuit section encompassed by a dotted line is a high electric potential side, low voltage circuit section (floating electric potential region) that is operated with the electric potential at the AC output terminal OUT as a reference. The electric potential at the AC output terminal OUT alternately changes to the electric potential at the common electric potential point COM and to the electric potential at the positive electrode side terminal Vdc of the high voltage main DC power supply, by the alternate turning-on and -off of the output IGBTs 17 and 18. Reference symbol E1 in the section of the circuit encompassed by the dotted line denotes an auxiliary DC power supply (also referred to as a driver power supply) supplying a voltage of 15V and for example, whose positive electrode and negative electrode are connected to a positive electrode line Vcc1 and the AC output terminal OUT, respectively. Although the auxiliary DC power supply E1 in FIG. 13 is a power supply with the electric potential of the AC output terminal OUT taken as a reference electric potential, if the IGBT 17 is a p-channel type, the auxiliary DC power supply may be provided with the electric potential of the positive electrode side terminal Vdc taken as the reference electric potential.
The NOT circuits 8 and 9 and circuits downstream (such as low pass filter circuits (LPF) 30 and 31, an RS flip-flop (RS latch) 15 and a driver 16) are operated using the auxiliary DC power supply E1 as a power supply. The load resistor circuit configured by the high voltage MOSFET 1 and the load resistor 3, or the load resistor circuit configured by the high voltage MOSFET 2 and the load resistor 4 is connected between the positive electrode line Vcc1 of the auxiliary DC power supply E1 and the common electric potential point COM and is operated using, as a power supply voltage, the voltage between the positive electrode line Vcc1 of the auxiliary DC power supply E1 and the common electric potential point COM. Because the electric potential of the AC output terminal OUT connected to the negative electrode of the auxiliary DC power supply E1 changes between the electric potentials of the common electric potential point COM and the positive electrode side terminal Vdc, the power supply voltage changes between E1+Vdc and E1 (in actuality, the cathode of a non-depicted, free wheeling diode is connected to each of the corrector sides of the IGBTs 17 and 18, whereby in a free wheeling mode of the free wheeling diode, sometimes causes the electric potential at the AC output terminal OUT to have a negative value on the order of several volts with respect to the common electric potential point COM).
Next, the operation of the level shift circuit will be explained. The on-signal 25 applied to the gate of the high voltage MOSFET 1 induces an electric current in the high voltage MOSFET 1, causing the electric potential to drop at the connection point of the load resistor 3 and the high voltage MOSFET 1. With the electric potential at the connection point becoming less than or equal to the threshold voltage of the NOT circuit 8, the NOT circuit 8 outputs a high level signal Hi. The high level signal Hi is applied to the set terminal S of the RS latch 15 through the LPF 30, is output from the output terminal Q of the RS latch 15, and applied to the gate of the IGBT 17 via the driver 16, turning-on the IGBT 17. At the same time (strictly speaking, at a time slightly before the IGBT 17 turns-on, to prevent an inter-arm short circuit), the IGBT 18 is turned-off by a signal transmitted through the driver 20 from the control circuit 61.
Subsequently, the off-signal 26 applied to the gate of the high voltage MOSFET 2 induces an electric current in the high voltage MOSFET 2, causing the electric potential to drop at the connection point of the load resistor 4 and the high voltage MOSFET 2. With the electric potential at the connection point becoming equal to or less than the threshold voltage of the NOT circuit 9, the NOT circuit 9 outputs a high level signal Hi that is applied to the reset terminal R 21 of the RS latch 15 through the LPF 31, causing the output of a low level signal Lo from the output terminal Q of the RS latch 15. The low level signal Lo is applied to the gate of the IGBT 17 through the driver 16 and turns-off the IGBT 17. At the same time (strictly speaking, at a time slightly after the IGBT 17 turns-off, to prevent an inter-arm short circuit), the IGBT 18 is turned-on by a signal transmitted through the driver 20 from the control circuit 61.
When the IGBT 18 is turned-off and the IGBT 17 is turned-on, an abrupt increase in electric potential dv/dt occurring at the AC output terminal OUT due to the switching, charges the drain-source capacitance of each of the high voltage MOSFETs 1 and 2. The charging currents at this time induce a voltage drop different from that resulting from the true on-signal or the true off-signal at the connection point of the load resistor 3 and the high voltage MOSFET 1 and at the connection point of the load resistor 4 and the high voltage MOSFET 2, causing a malfunction of the RS latch 15, whereby the IGBT 17 may be turned-on inadvertently, causing an inter-arm short circuit of the bridge circuit or an unnecessary turning-off of the IGBT 17.
Similar abnormal voltage drops may occur at the connection point of the load resistor 3 and the high voltage MOSFET 1 and at the connection point of the load resistor 4 and the high voltage MOSFET 2 during times other than at the switching of the IGBTs 17 and 18 and may be caused by external noise. The low pass filter circuits 30 and 31 are inserted to prevent such malfunctions in the RS latch 15 by performing a role of eliminating, as abnormal signals, input signals having small pulse widths (higher frequencies) caused by switching or external noise.
As shown in FIG. 13, the IGBT 17 and 18 are turned-on and-off by using the on-signal 25 and the off-signal 26, which are pulse signals, because to carry out high speed switching of AC signals output from a circuit such as a PWM inverter, the frequency of a carrier for turning-on and -off an output switching device is preferably increased, namely, a level shift circuit is preferably operated at a high speed. Increasing the frequency of the carrier for turning-on and -off the switching device is related to increasing the frequency of the PWM inverter, which has a merit of making it possible to downsize coils on the power supply circuit board in the inverter system, thereby enabling reduction of the area of the power supply board.
Therefore, to operate a level shift circuit at a high speed, it is necessary to let relatively large electric current flow in the high voltage MOSFETs 1 and 2 of a level shift circuit. However, as shown in the section encompassed by the dotted line in FIG. 13, in the high electric potential side, low voltage circuit section, whose reference electric potential changes, particularly in the case when the reference voltage is high, losses due to the relatively large electric current increase. For example, when each of the high voltage MOSFETs 1 and 2 is turned-on with a signal that is input to the gate and generated by a pulse generator, if an on-current of 10 mA flows in each of the high voltage MOSFETs 1 and 2, the voltage of the positive electrode side terminal Vdc of main DC power supply is 400V, and the duty cycle for the turning-on and -off of the high voltage MOSFETs 1 and 2 is 10 percent on average; the average loss for each of the high voltage MOSFETs 1 and 2 is on the order of 0.4W.
Moreover, Patent Document 2 describes a high voltage IC having a high side gate driver and a level shift circuit of a power device. In the high voltage IC, a high voltage MOSFET for the level shift and an isolated island region (floating electric potential region) are connected by wiring formed, through an insulating film, on a semiconductor substrate. Thus, between the high voltage MOSFET and the isolated island region, the semiconductor substrate is exposed from an opening selectively formed in the insulating film and this exposed region is connected to the wiring. When a high voltage is applied to wiring connecting the high voltage MOSFET and the isolated island region, a depletion layer expanding from the high voltage MOSFET and a depletion layer expanding from the isolated island region meet, increasing the electric potential of the region where the substrate beneath the wiring is exposed.